P4 Electricity Flashcards
Diode
A non-ohmic conductor that has a much higher resistance in one direction(its reverse direction) than in its forward direction
Light-depending resistor
A resistor whose resistance depends on the intensity of the light incident on it
Light-emitting diode
A diode that emits light when it conducts
Ohm’s law
The current through a resistor at constant temperature is directly proportional to the potential difference across the resistor
Parallel
Components connected in a circuit so the potential difference is the same across each one
Potential difference
A measure of the work done/energy transferred to the lamp by each coulomb of charge that passes through it
Resistance(equation)
Potential difference/Current
Series
Components connected in a circuit in such a way the same current passes through them
Thermistor
A resistor whose resistance depends on the temperature of the thermistor
Electrical current
A flow of electrical charge
Potential difference
The driving force that pushes the charge round
How does the resistance across a component affect the current flowing through a component?
The greater the resistance across a component, the smaller the current that flows(for a given potential difference across the component).
Charge flow =
Current x time(seconds)
Potential difference =
Current x resistance
What measures the current through a test wire?
The ammeter
What measures the potential difference across the test wire?
The voltmeter
How should ammeters always be placed?
In series with whatever you’re investigating
How should voltmeters always be placed in a circuit?
The voltmeter must always be placed in parallel around whatever you’re investigating
Required practical(to investigate the factors affecting resistance)
1) Attach a crocodile clip to the wire level with 0cm on the ruler.
2) Attach the second crocodile clip to the wire e.g. 10 cm away from the first clip. Record the length of the wire between the clips.
3) Close the switch, then record the current through the wire and
the potential difference across it.
4) Open the switch, then move the second crocodile clip, e.g. another 10cm along the wire. Close the switch again, then record the new length, current and potential difference.
5) Repeat this for a number of different oengths of the test wire.
6) Use your measurements of current and potential difference to calculate the resistance for each length of wire.
7) Plot a graph of resistance against wire length and draw a line of best fit.
What should be the results of the practical investigating the factors affecting resistance?
The graph should be a straight line through the origin - showing the longer the wire, the greater the resistance.
Possibly systematic errors(in resistance practical).
The first clip isn’t attached exactly at 0cm, so all the length readings are a bit out(seen if the graph doesn’t go through the origin).
How to ensure accuracy(in resistance practical).
1) Thin wire will give best results.
2) Ensure the wire’s as straight as possible so the length measurements are accurate.
3) The wire may heat up during the experiment, which will affect its resistance. Leave the switch open for a bit between readings to let the circuit cool down.
At a constant temperature, how does current affect potential difference?
At a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it.
Examples of ohmic conductors
Wire(at constant temperature)
Resistor(at constant temperature)
In which components, does the resistance change as the current through them changes?
A diode or a filament lamp
How does resistance change when an electrical charge flows through a filament lamp?
When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament(which heats up). Resistance increases with this increased temperature.
I-V Characteristic
Refers to a graph which shows how the current flowing through a component changes as the potential difference across it is increased.
Linear components
Have an I-V characteristic that’s a straight line
Examples of linear components
Any ohmic conductors(e.g. a fixed resistor)
Non-linear components
Have a curved I-V characteristic
Examples of non-linear components
A filament lamp/diode
I-V characteristics in a filament lamp
As the current increases, the temperature of the filament increases, so the resistance increases, meaning less current can flow per unit potential difference(hence the curve).
I-V characteristics in a diode
Current will only flow through a diode in one direction - the diode has very high resistance in the reverse direction.
How does bright light affect the resistance of a light dependent resistor?
The resistance falls
How does dark light affect the resistance of a light dependent resistor?
The resistance is highest
Applications for light dependent resistors
Automatic night lights
Outdoor lighting
Burglar detectirs
How do hot conditions affect the resistance in a thermistor?
The resistance drops
How do cool conditions affect the resistance in a thermistor?
The resistance goes up
Uses of thermistors
Useful temperature detectors e.g. car engine temperature sensors and electronic thermostats
Sensing circuits
Sensing circuits can be used to turn on/increase the power to components depending on the conditions they are in
Example of sensing circuit(fan)
1) The fixed resistor and fan always have the same potential difference across them(they’re connected in parallel).
2) The potential difference of the power supply’s shared out between the thermistor and loop made up of the fixed resistir and the fan according to their resistances - the bigger a component’s resistance, the more potential difference it takes.
3) As the room gets hotter, the resistance of the thermistor decreases and takes a smaller share of the potential difference from the power supply, so the potential difference across the fixed resistor and fan rises, making the fan go faster.
Resistance in ohmic conductors?
The resistance in ohmic conductors doesn’t change with the current.
How are components connected in series circuits?
The different components are connected in a line, end to end, between the +ve and -ve of the power supply
What happens if you remove/disconnect one component in a series circuit?
The circuit is broken - all stops
How is potential difference shared in a series circuit?
The total potential difference of the supply is equal to all the potential differences round a series circuit added up
What is current like in a series circuit?
In series circuits, the same current flows through all components.
What is resistance like in a series circuit?
The total resistance of two components is just the sum of their resistances in series circuits
What are the potential differences of a group of cells like in a series circuit?
The total potential difference between a group of cells is the potential difference of each cell added together
How are components connected in parallel circuits?
In parallel circuits, each component is separately connected to the +ve and -ve of the supply
What happens when you remove/disconnect one of the components in a parallel circuit?
It will hardly affect the other components at all
Uses of parallel circuits
How most things must be connected e.g. in cars and in household electrics
What is potential difference like in a parallel circuit?
The potential difference is the same across all components in a parallel circuit
What is current like in a parallel circuit?
In parallel circuits, the total current flowing around the circuit is equal to the total of all the currents through the separate components
What is resistance like in a parallel circuit?
If you have two resistors in a parallel circuit, their total resistance is less than the resistance of the smallest of the two resistors, as this decreases the charge that can flow through the circuit
How to investigate the impact adding resistors has on resistance in a series circuit
1) Find at least four identical resistors.
2) Make a note of the potential difference of the battery after building a circuit with one of the resistors.
3) Measure the current through the circuit using the ammeter. Use this to calculate the resistance of the circuit using resistance = potential difference/current.
4) Add another resistors, in series with the first and measure the current again through the circuit, using this and the potential difference of the battery to calculate the resistance of the circuit.
5) Repeat these steps until you’ve added all these resistors.
How to investigate the impact adding resistors has to the resistance in a parallel circuit.
1) Build a circuit using one of four identical resistors and a battery(noting the potential difference).
2) Measure the total current through the circuit and calculate the resistance of the circuit using resistance = potential difference/current.
3) Next, add another resistor, in parallel with the first.
4) Measure the total current through the circuit and use this along with the potential difference of the battery to calculate the overall resistance of the circuit.
5) Repeat these steps until you’ve added all of your resistors.
How does adding resistors affect resistance in series?
Adding resistors in series increases the total resistance of the circuit.
How does adding resistors affect resistance in parallel circuits?
When you add resistors in parallel, the total current throughnthe circuit increases - so the total resistance of the circuit has decreased.
Why does the resistance of the bulb increase as the potential difference across the bulh increase?
Because an increasing potential difference increases the temperature of the bulb
How a student would use the circuit to take the readings necessary to determine the resistance of resistor R?
1) Switch on the circuit.
2) Read both the ammeter and voltmeter.
3) Adjust the variable resistor to change the current and take further readings.
4) Draw graph of V against I
5) Resistance = Potential difference/Current
What effect does a current through a wire have on the wire?
Makes it hot/warm
When investigating how the current through a thermistor depends on the temperature of the thermistor, what is one way the investigation could be changed to give a wider range of temperatures?
Use hotter water
Another function of variable resistor
To change current
1A=
1000Ma
